As can be seen by reference to the following U.S. Pat. Nos. 4,539,845; 4,283,956; 4,307,610; 4,446,733; and 4,389,891, the prior art is replete with myriad and diverse resonance measuring and testing apparatus.
Molimar, U.S. Pat. No. 4,539,845, describes the mechanical sensing of the natural frequency of an object under fatigue testing, electrically coupling the sensed frequency to a mechanical input device so that the object is kept at resonance but controlling amplitudes to fixed values, thereby reducing the testing time and forces required for fatigue testing compared with the other methods, wherein the amplitude of vibration of the tested object (e.g., engine and motor components) is kept to a predetermined set point value.
Okubo, U.S. Pat. No. 4,446,733, like Molimar, uses a combination of mechanical sensing, electric coupling, and mechanical input to measure and maintain resonance in structural materials for the purposes of stress relieving, fatigue testing and non-destructive load testing.
Fournier, U.S. Pat. No. 4,389,891, in a manner similar to Moilmar and Okubo, also uses a combination of mechanical sensing, electric coupling, and mechanical input to measure the natural frequencies in turbine and compressor vanes and propeller blades.
In addition, Leupp, U.S. Pat. No. 4,307,610, uses a combination of machanical sensing, electric coupling, and mechanical input to maintain resonance in order to measure crack propagation in samples for assessing the fatigue behavior of a material or a component; and Lechner, U.S. Pat. No. 4,283,956 induces resonance to detect and indicate the onset of cracking in articles subjected to dynamic loading.
While all of the aforementioned prior art patents are more than adaquate for the basic purpose and function for which they have been specifically designed, these prior art methods and apparatus are ultimately aimed at preventing fatigue type destruction and have overlooked the fact that even though resonance can be a highly destructive force, that destructiveness can be used for useful purposes.
Resonance is an extremely powerful phenomena. Major man-made structures, designed to be indestructible, have been destroyed by relatively insignificant forces, which by chance have been applied at resonant frequencies. All objects and structures have resonant frequencies, some of which can be sufficiently "damped" to be almost undetectable.
The destructive power of resonance is witness to the well known Army instruction "break step on bridges" and is evidenced by that most famous bridge failure at Tacoma, Wash., captured on film in 1940, wherein a 2,800 foot span of two lane bridge literally "blew down" in a 40 mph wind.
While the body of knowledge on resonance is extremely large, there have been vary few attempts to use the destructive power of resonance for useful work in the mining industry.
The work that has been done appears to have concentrated on the ultrasonic frequency range, i.e., above 20,000 cycles/second, whereas, experimental field data on three rock types indicates that lower frequencies, under 4,000 Hz, are more applicable.
Resonant frequencies are those that a solid body naturally assumes during relaxation from an energized state to an unenergized state. The lowest frequency at which a body freely vibrates is called the primary frequency. Other resonant frequencies are called harmonics. When bodies are excited, deliberately or by accident, at their resonant frequencies, very small forces can display seemingly disproportionate and devastating effects.
The application of energy to excite resonant frequencies is restricted by basic underlying principles. Vibration can be represented by a simple pendulum, such as a ball suspended from a string. To initiate the pendulum motion, the ball is displaced to one side of the quiescent position of the pendulum. Once the ball is released, the most effective phase of the pendulum swing to apply energy to increase amplitude occurs between the release of the ball and the arrival of the ball at the quiescent position. As the ball passes through the quiescent position, the positive force of the ball diminishes to the point where the ball stops and swings back towards the release position wherein the reverse travel of the ball is always active against the initial direction of the swing.
Applying this example to the principles of resonance, it is the amplitude of vibration exceeding the elasticity constant which breaks solid objects. The objective in applying resonance for destruction is therefore to maximize the swing. It can be seen that the most effective time to apply energy to the pendulum is the first one-fourth of one cycle. To maximize amplitude, under no circumstance can the energy pulse be greater than one-half of one cycle. This is the first of two basic principles. Pulse time (t) must be less than 1/.sub.2f (ideally, 1/.sub.4f) where f =primary frequency, in cycles per second.
Again, using the pendulum, it is apparent that the energy pulse can be applied every swing (cycle) every second swing, or every third swing etc., it cannot be applied twice per swing. This is the second principle. The frequency of the pulse is either f or f divided by an integer. It cannot exceed f.
While these two principles are simple, maintaining their integrity in practice is not. Indications are that the applicable frequency range is between 200 and 5,000 cycles each second. Physically pulsing energy at these high cyclical rates is difficult enough and is compounded by the requirement for absolute accuracy. If the pulse frequency is out by even one cycle per second, then for half of every second the pulses act against resonance.
Unlike the simple sine-wave type motion of a swing, rock present a much more complex phenomena. The apparent resonant frequency of a particular rock may be expected to be effected by at least the following: the mass of the rock, the rock material, the circumstances of the rock (i.e., free standing, partially embedded, etc.), discontinuities--joints and fractures, the point of measurement, and, the point of excitation.
However, provided that: the points of excitation and measurement do not change, the input force frequency exactly matches the measured frequency or the measured frequency divided by an integer, the input force waves are supportive in phase, and the amplitude of vibration does not return to zero between pulses, then the rock will be in resonance.
If the amplitude is increased to the point where the measured resonant fequency is changed, then destructive work has been accomplished. This of course may not break the rock--it may merely have altered the circumstances of a fracture or joint plane. To effectively achieve breakage, not only must the amplitude of the resonant vibration be sufficient, but any change in measured output frequency must immediately be reflected in the input frequency.